1. Field of the Invention
The present invention relates to an adjacent and alternate channels power measurement apparatus and, more particularly, to an adjacent and alternate channels power measurement apparatus for measuring adjacent and alternate channels power in TDMA (time-division multiple access) communications using a burst signal.
2. Description of the Related Art
As digital communication systems using TDMA communications, the PDC system (Japan digital mobile telephones), the NADC system (U.S. digital mobile telephones), the GSM system (Europe digital mobile telephones), the PHP system (Japan digital cordless telephones), and the like are known.
A digital communication system has a plurality of channels to perform transmission/reception of data between a base station and a plurality of mobile stations. For example, as shown in FIG. 5, channels 1, 2, 3, and 4 which respectively have center frequencies F1, F2, F3, and F4, and a frequency bandwidth AF are assigned.
A communication between the base station and a given mobile station is performed using a burst-pattern signal (to be referred to as a burst signal hereinafter) which is turned on and off at a predetermined time interval. For example, a burst signal in which a duration (T.sub.S) including a carrier C and a duration (T.sub.F -T.sub.S) including no carrier repetitively appear at a predetermined time interval (T.sub.F) is used, as shown in FIG. 4.
In such a communication system, a wide distribution of the spectrum characteristics of a channel signal in each channel may adversely influence other channels, and may pose problems.
More specifically, when the distribution of the spectrum characteristics of a carrier (to be referred to as the carrier of a main channel) of a specific channel (a channel having the carrier frequency of a signal to be measured; to be referred to as a main channel hereinafter) is broad, the electric power of the carrier of the main channel leaks to adjacent and alternate channels, and appears as noise in these channels. For this reason, it becomes difficult for the receiving side of each of the adjacent and alternate channels to separate the leakage carrier of the main channel and to receive only the carrier of the own channel.
FIG. 5 shows an example of a wide distribution of the spectrum characteristics of a carrier C2 (the carrier of the main channel) of channel 2 (main channel). In this case, leakage of the spectrum characteristics (b) to channels 1, 3, and 4 (adjacent and alternate channels) is larger than that of the spectrum characteristics (a), and adversely influences these channels.
Therefore, in order to prevent the above-mentioned problem, the transmitting side of each channel must limit the distribution of the spectrum characteristics of a channel signal included in the burst signal.
For this purpose, a measurement apparatus which receives the carrier of each channel, and can quantitatively measure the distribution of the spectrum characteristics of the received carrier as adjacent and alternate channels power is required.
As a conventional measurement apparatus for measuring the adjacent and alternate channels power, an apparatus described in, e.g., IEC STANDARD.Publication 489-2, 1978, which measures a continuous signal in place of a burst signal, is known.
FIG. 6 is a block diagram showing this prior art.
A signal to be measured (continuous signal) input from a terminal 1 is mixed with a local signal from a local oscillator 22 by a mixer 21, and is converted into an IF signal of a predetermined frequency. The bandwidth of the IF signal is limited by a bandpass filter 23 having prescribed bandpass characteristics (bandwidth .DELTA.F). Thereafter, the IF signal is envelope-detected by a level detector 24. The detected signal is converted into digital data by an A/D converter 25, and the digital data is input to a power arithmetic operation unit 4. A frequency switching unit 3 switches and sets the frequency of a local signal to be output from the local oscillator 22, and outputs the switching information to the power arithmetic operation unit 4. Note that the frequency of the local signal to be output from the local oscillator 22 varies depending on a channel to be selected.
The power arithmetic operation unit 4 calculates a time average of the levels of the digital data on the basis of the digital data of the detected signal output from the A/D converter 25, thus calculating electric power for every channel. Note that the mixer 21, the local oscillator 22, the bandpass filter 23, the level detector 24, and the A/D converter 25 constitute a signal processing unit 2.
The frequency setting method of the local oscillator 22 and the waveform of the frequency component of each channel selected by the bandpass filter 23 will be described below with reference to a case wherein a continuous signal of the carrier C2 having the spectrum characteristics (b) shown in FIG. 5 is input as a signal to be measured.
In order to select the frequency component of a given channel from a signal to be measured shown in FIG. 7A (the carrier C2 including the frequency components of channels 1 to 4), a frequency higher (or lower) by an IF frequency (F0) than the center frequency (F1 to F4) of the channel is set in the local oscillator 22. For example, in the case of channel 2, a frequency F2+F0 (or F2-F0) is set. As a result, even when the frequency band (channel) to be selected is changed in turn, the center frequency of the IF signal output from the mixer 21 can be a constant frequency (F0).
FIGS. 7B, 7C, and 7D respectively show the waveforms of IF signals of the respective channels selected by the bandpass filter 23. As can be seen from FIGS. 7B, 7C, and 7D, the waveform level of each channel corresponds to the level of the spectrum characteristics of each channel. Upon reception of the detected signal (digital data) corresponding to the waveform level, the power arithmetic operation unit 4 calculates a time average, thus calculating electric power for every channel.
In this case, since channel 2 is the main channel, electric power calculated for each of channels 1, 3, and 4 will be referred to as adjacent and alternate channels power hereinafter.
However, when the adjacent and alternate channels power of the above-mentioned burst signal is measured using the above-mentioned conventional measurement apparatus, the following problems are posed.
When a burst signal which is shown in FIG. 8A and has the spectrum characteristics (b) in FIG. 5 is input as a signal to be measured, the waveforms of the detected signals of the respective channels which are envelope-detected by the level detector 24 and are output therefrom are as shown in FIGS. 8B, 8C, and 8D.
More specifically, upon turning on/off (corresponding to (c) in FIG. 8B) of the carrier C2, large power ((d) in FIGS. 8C and 8D) is generated in the adjacent and alternate channels. In a predetermined period ((e) in FIG. 8B) of an ON duration as well, the adjacent and alternate channels power ((f) in FIGS. 8C and 8D) is generated. The former power (d) will be referred to as a peak value of the adjacent and alternate channels power hereinafter, and the latter power (f) will be referred to as a steady value of the adjacent and alternate channels power hereinafter.
It is required for the adjacent and alternate channels power generated as described above that the peak value and the steady value be separately evaluated (for example, the standards of U.S. digital mobile telephones; EIA/TIA/IS-55 December 1991).
However, in the above-mentioned measurement apparatus for a continuous signal, i.e., the conventional technique for calculating electric power as a time average of the detected signal (digital data), it is impossible to separately measure the peak value and the steady value of the power.